deadline vs due date

deadline

noun
  • A time limit in the form of a date on or before which something must be completed. 

verb
  • To render an item non-mission-capable; to ground an aircraft, etc. 

due date

noun
  • The date by which a task is supposed to be completed. 

  • The date on which a pregnant woman is expected to give birth. 

  • The date by which a book (or other item) borrowed from a library must be returned. 

  • The date of payment in financial arrangements. 

How often have the words deadline and due date occurred in a corpus of books? (source: Google Ngram Viewer )